The present invention relates to a method for the manufacture of a high-tensile strength optical waveguide having a plastic layer of the epoxy-acrylate type located on the lightguide fiber thereof through application of radiation-hardenable prepolymers to the lightguide fiber immediately following the fiber-drawing process.
Optical waveguides consist essentially of glass fibers which have an index-of-refraction profile such that incident light is guided in them, following all curves of the fibers. By reason of this property, these waveguides may serve as a transmission medium. However, the use of optical glass waveguides in communication systems, for example, for information transmission, requires consideration of two intrinsic problems; namely, realizing high strength inherent in the fiber itself and avoiding an increase in attenuation caused by micro-bending of the fiber.
To eliminate the problems mentioned, the lightguide fibers are typically coated with suitable plastic materials. Non-reactive coating materials have been utilized, such as polyvinylidene fluoride and polymethylsiloxane ("Siemens Forsch.-u. Entwickl.-Ber." Vol. 6, 1977, No. 5, pp. 314 to 319), polyesters, polyimides and polyester imides (German Offenlegungsschrift No. 27 29 648), as have reactive coating materials, such as epoxides and polyurethanes ("Siemens Forsch.-u. Entwickl. Ber." Vol. 7, 1978, No. 3, pp. 158 to 165).
Heretofore, application of the plastic coating for increasing the tensile strength and for protecting the surface of the glass from mechanical influences and damage, has preferably been performed by a varnishing technique, the plastic being applied from solution by means of a wick or a roll or by an immersion technique. The varnishing technique permits an application of only a few .mu.m, i.e., of about 5 .mu.m, of the coating in a technically satisfactory manner per step. However, experience shows that a layer applied directly to the lightguide fiber must have, on the average, a thickness of at least about 20 .mu.m in order to be effective. Accordingly, several successive, time-consuming coating operations are necessary with the varnishing technique to obtain layers which increase the tensile strength or offer effective mechanical protection.
Application from solutions further requires long drying sections and high drying temperatures in order to obtain acceptable production speeds, as well as elaborate apparatus for drawing-off the solvent vapors in order to prevent pollution of the environment. In addition, solvent occlusions are detrimental to the coating. It is also difficult, due to the surface tension conditions between the coating material and the substrate, to obtain uniform coating of the plastic on the fiber since the so-called "pearl-string" effect can only be reduced, but not prevented, if the coating material becomes temporarily of low viscosity when it enters the drying zone.
These same considerations and difficulties apply to coating with reactive, e.g., additively cross-linking, resin systems if they are applied from solutions; however, even solvent-free systems exhibit the pearl-string effect. A further aggravating circumstance in additively cross-linking systems is the lack of constant viscosity due to the fact that the components continue to react, especially in the coating device. Here again, uniform coating cannot be assured.
In addition, the speed-determining factors in production are the drying speed and the reaction rate of the coating materials as well as the thickness of the plastic layer. Raising the temperature in the drying zones as a means of shortening the drying time can only be practiced to a limited degree before adverse effect on the surface quality of the varnish layer results.
Plastic layers can also be produced by extrusion of a firmly adhering jacket directly on the fiber surface. This method, however, is often accompanied by additional attenuation due to micro bands. Coating from a hot melt remains limited to use of only a very few materials with low melt viscosity, and temperature control in the process is critical. Other possible coating methods, such as those which are based on the spraying technique, are not suitable because of the small size of the substrate, i.e., the lightguide fiber. Another possibility of coating lightguide fibers, discussed in the technical literature, is the electrostatic application of jacketing materials in powder form. This method, however, requires a fiber which either is itself electrically conductive or is made electrically conductive and with which, in addition, contact must be maintained during the coating process.
Besides these problems relating merely to the application of the coating, problems regarding the solidification of the material, once applied, also arise. Solidification, which is generally accomplished by evaporation of solvents or by a chemical cross-linking reaction, requires more technical means and requires extended periods of time. The time factor, however, obviously is important for an economical production process. In addition, there exist problems regarding the adhesion of the plastic to the glass surface of the fibers, good adhesion of the coating material being necessary to obtain high tensile strength. For this purpose, adhesion agents are, therefore, used. However, with direct application of an adhesion agent to the fiber surface, which is generally the most effective form of application, an additional operation is required. In addition, mechanical damage to the virgin glass surface, especially by the coating cuvette, may be encountered using such a procedure.
From U.S. Pat. No. 4,099,837, an optical waveguide is known, on the glass fiber of which is applied a polymer coating, of the polymerization product of a prepolymer mixture, for increasing the tensile strength, which prepolymer mixture is obtained through reaction of acrylic acid with a mixture of an aliphatic and an aromatic diglycidyl ether in the equilibrium ratio 0.4:1.0. The mixture also contains a UV-stabilizer for hardening the prepolymer with ultraviolet light. In addition, the coating material preferably also contains an adhesion agent of the silane or titanate type. In addition to the requirement for an adhesion agent, a distinct disadvantage in these known optical waveguides is that the plastic layer of the epoxy-acrylate type contains free epoxy groups, i.e., reactive groups, which not only can cause stickiness but is also objectionable from a toxological point of view.